Ferricretes and deep weathering profiles of the Puzzler Walls, Charters Towers, north Queensland

The Charters Towers district in semi-arid tropical north Queensland has large areas of transported and residual cover. It is host to several major gold and base-metal deposits and has a long history of mineral exploration, but there have been few studies of its regolith. As future exploration will target mineralisation under cover, an understanding of the regolith is essential for interpretation of geochemical and geophysical data. Transported regolith presents further complications. The Puzzler Walls, about 10 km east of Charters Towers, are a group of mesas capped by siliceous and ferruginous duricrusts developed on thick units of deeply weathered Tertiary sediments and Palaeozoic granitoids. The granitoids host mesothennal quartz-vein type gold mineralisation, commonly with associated sulphides, particularly galena. Analysis of ferruginous material from the mesas reveals two main types of iron accumulations: an older ferruginous gravel developed from the weathering of granodiorite, and a younger nodular ferricrete overlying Tertiary/Quaternary sediments. These materials are distinguished on the basis of their position in the landscape, micromorphology (petrology), geochemistry, and mineralogy. The older (Tertiary?) ferricretes have developed in situ on the granodiorite. They consist of ferruginous aggregates of the underlying mottled saprolite, formed by the relative accumulation of iron, as other, more soluble, elements were leached out. The younger (Tertiary-Quaternary?) nodular ferricretes consist of reworked or transported iron nodules coated by several generations of goethite rinds. These nodular ferricretes are interpreted as having formed by the accumulation of iron, precipitated from groundwater on valley sides or oxidation fronts. The geochemistry of both ferricrete types reveals enrichment of transition metals, some rare-earth elements, and elements commonly associated with gold mineralisation in the Charters Towers district (Pb, Sb, As). The last may be useful pathfinders in exploration. It still needs to be established whether this enrichment results from proximity to underlying mineralisation .